Heterogeneous Chlorine Chemistry in the Atmosphere: Insights From Modeling and Experiments

Heterogeneous Chlorine Chemistry in the Atmosphere: Insights from Modeling and Experiments.

Background

            To date, numerous studies using both computational and experimental methods have shown that the presence of gas phase atomic chlorine radicals (Cl^•) can influence tropospheric photochemistry.  Specifically, Cl^• participates in the ozone production cycle in a manner analogous to the hydroxyl radical (OH^•), and elevated concentrations can lead to increases in ozone production rates.  Cl^• radicals are produced from the photolysis of a gas phase precursor such as Cl₂ or HOCl.  Impacts on ozone production resulting from anthropogenic point source emissions of these gas phase Cl^• precursors tend to be highly localized due to rapid rates of photolysis during the day.

            Aside from direct emission from anthropogenic sources, gas phase chlorine species can be produced through heterogeneous mechanisms involving chloride aerosols.  One such species is nitryl chloride (ClNO₂), which was initially observed in ambient air in 2006 during a study along the Gulf Coast of the U.S.  Additional observations have also been made in Boulder, CO,  Alberta, Canada and off the coast of California, near the Los Angeles Basin.  Multiple studies investigating ClNO₂ production have determined that the responsible mechanism is the reactive uptake of N₂O₅ onto aqueous chloride aerosols.  N₂O₅, which is formed from the gas phase reaction between NO₂ and NO₃, photolyzes rapidly and therefore only builds up to significant concentrations at night.  The nocturnal conversion of N₂O₅ into ClNO₂ results in the rapid production of Cl^• when ClNO₂ photolyzes at sunrise.  Through gas phase reactions initiated by Cl^•, this can increase the rate of oxidation for volatile organic compounds (VOCs) and thus increase the rate of ozone production.  The heterogeneous uptake and dissociation of N₂O₅ has been determined to be the rate limiting step, and the full mechanism is shown below.

NO_2,g + NO_3,g               ↔       N₂O_5,g                                     (1)

N₂O_5,g                           ↔       N₂O_5,aq                                    (2)

N₂O_5,aq                          ↔       NO₂⁺_aq + NO₃^-_aq                     (3)

NO₂⁺_aq + H₂O_aq            →        H₃O⁺_aq + HNO_3aq                    (4a)

NO₂⁺_aq + Cl^-                  →        ClNO₂ + H₂O_aq                        (4b)

NO₃^-_aq + H⁺                   ↔       HNO₃⁺_aq                                  (5)

Another heterogeneous mechanism of interest involves the production of Cl₂ from chloride aerosol particles.  Previous studies have suggested a direct reaction between O₃ and particulate chloride.  Other studies have suggested a surface reaction between particulate chloride and OH^•.  These mechanisms were formulated in response to experimental observations of gas phase Cl₂ when aqueous NaCl and O₃ were present together.  The reaction has been reported to occur both in the presence of light as well as in the dark.  The mechanism for the reaction between OH^• and particulate chloride can be represented as follows, where 2OH•••Cl^- represents an intermediate species that forms at the particle surface.

OH_g + Cl^•-_aq,surface          →        OH•••Cl^•-_aq,surface                 (6)

2OH•••Cl^-_aq,surface           →        Cl_2,g + 2OH^-_aq                         (7)

Cl₂ concentrations up to 150 ppt_v have been detected in marine air at night, suggesting a nocturnal 330 ppt_v day^-1 Cl₂ source.  Additionally, detection of Cl₂ in both coastal marine and coastal urban air has been reported.  Therefore, the determination of an exact mechanism and quantification of the rate would be useful in predicting the impact Cl₂ production from chloride aerosols on tropospheric chemistry.  This is particularly relevant in coastal regions where chloride particles are typically present in large quantities.  The mechanism involved in reactions 6 – 7 is the focus of the experimental results presented here.

Methods and Discussion

This work presents both computational and experimental investigation of the heterogeneous processes described above.  An overview of a sensitivity analysis of the physical parameters involved in the mechanism described by reactions 1 – 5 will be discussed.  The parameters investigated in the analysis include the reactive uptake coefficient, ClNO₂ yield, particle surface area, and gas phase concentrations of VOCs and NO_x.  The sensitivity analysis was carried out through photochemical box modeling using the SAPRC software and the Carbon Bond 5 mechanism.  The focus of this sensitivity analysis was on the effects of variations in parameter values on the production of ClNO₂ and subsequent impacts on ozone production.  The results obtained from simulations using various parameter value combinations were compared to a base case modeling scenario in which all heterogeneous reactions were absent.  In order to quantify differences in O₃ production, the peak O₃ concentration from each simulation was used as a point of comparison.  An analysis of parameter values reaching the upper limits reported by the literature was undertaken. 

Results from the sensitivity analysis indicate that ClNO₂ chemistry can potentially change peak O₃ concentrations by -10.5% – 27%.  The availability of NO_x was found to play an important role.  The presence of additional NO_x during nocturnal hours resulted in greater production of N₂O₅ and thus higher concentrations of ClNO₂.  However, NO_x emissions after sunrise reduced the impact of N₂O₅-ClNO₂ chemistry on O₃ production.  Additionally, the presence of large concentrations of VOCs that are highly reactive with the NO₃ radical significantly reduced the impact of N₂O₅-ClNO₂ chemistry on ozone production.  Figure 1 shows the combined influence of variations in the reactive uptake (γ) and ClNO₂ yield (Y_ClNO2) on peak ozone concentrations as compared to the base case scenario.  Further discussion of details on the impacts of ClNO₂ chemistry with respect to O₃ production and NO_x cycling within the atmosphere will be included in the presentation.

Figure 1: Percentage peak O₃ increases in over the base case scenario (no heterogeneous reactions) for various combinations of the ClNO₂ yield and reactive uptake parameter values.

Experimental work investigating the heterogeneous mechanism described in reactions 6 – 7 will also be covered.  Specifically, results from environmental chamber experiments involving the exposure of NaCl aerosol particles to various gas mixtures representative of conditions present in the atmosphere, including HO_x, NO_x and O₃ will be presented.  Experiments were carried out in a 10 m³ reaction chamber equipped with UV lights which induce photochemistry.  Chemical Ionization Mass Spectrometry (CI-MS) utilizing iodide as the reagent ion was used to quantify and identify gas phase species produced by the mechanism.  Aerosol population size distributions were monitored using a Scanning Electrical Mobility System (SEMS). 

Experiments were carried out by generating and injecting NaCl aerosol particles into the chamber along with photolytic precursors of the hydroxyl radical.  The mixture was then exposed to UV radiation in order to initiate photochemistry and hydroxyl radical production.  Resulting gas phase Cl₂ concentrations were monitored using iodide CI-MS.  Various initial conditions were used to test the impact of several factors on the overall efficiency of the mechanism in producing gas phase chlorine.  One factor is the impact of variations in relative humidity levels on the efficacy of the mechanism.  Another factor is the variation of the composition of the bulk aerosol.  The impact of the presence or absence of elevated O₃ concentrations is also examined.  Resulting variations in overall Cl₂ production due to these factors will be compared and discussed.